More details came to light Tuesday about the
weekend attack that compromised a German Web advertising company's server and infected users who visited Web sites run by the firm's clients with the MyDoom/Bofra worm.

According to a statement released last Monday night by Falk eSolutions, the attack began early Saturday morning when an attacker managed to infiltrate the server and place code that redirected ad requests to a third-party site.

That site was http://search.comedycentral.com, which had also been compromised by the attacker. Neither Falk nor the SANS Institute's Internet Storm Center, which tracked and helped investigate the attack, have any idea when Comedy Central's site has cracked. As of mid-morning Tuesday, Comedy Central's search site was offline.

"It looks like Comedy Central and perhaps some other sites were compromised first, followed by Falk. Then, Falk's site was configured to redirect visitors to Comedy Central," said Marcus Sachs, the director of the Storm Center, in a statement.

Falk continued to play down the extent of the problem, saying potential redirects to the exploit code on the Comedy Central server were less then 2 percent of the ad requests from European client sites, and under 0.1 percent of those from U.S. clients during the six-hour span of the attack.

To some extent, Falk put the blame on Microsoft, whose Internet Explorer 6.0 browser was exploited by the attack when the redirected requests downloaded the MyDoom/Bofra worm instead of ads to PCs. The bug in IE, dubbed the IFRAME vulnerability, has not yet been patched,

"Users should consider using an alternate browser to Internet Explorer (such as Opera or Firefox) or upgrading their Microsoft operating system [to Windows XP SP2], Falk said in its statement.

Falk also said that it considers the hack a criminal matter, and has notified local authorities in Germany.